1. Field of the Invention
The present invention relates to a method and a device for charging a photosensitive body in contact with a charging member in an electrophotographic apparatus such as a laser printer and a copying machine.
2. Description of the Related Art
An electrophotographic apparatus, for example, a laser printer includes a charging device, an exposure device having a laser beam emitter, a developing device, and a transfer device which are arranged around a photosensitive body. In the laser printer, when printing is performed, the charging device is operated to uniformly charge the photosensitive body, and then the exposure device is operated to scan the photosensitive body with a laser beam having recording information, thereby forming an electrostatic latent image thereon. After this, the developing device is operated to stick the toner on the electrostatic latent image, and the transfer device is operated to transfer a toner image formed by the operation of the developing device onto paper.
There are two methods for charging the photosensitive body using the charging device of the laser printer. The first method is executed by corona discharge caused by a scorotron, and the second method is executed by bringing a member such as a charging roller, a charging brush, and a charging blade into contact with the photosensitive body.
The first method requires a high voltage power supply of 5 to 10 KV and generates a great deal of ozone due to the corona discharge. The ozone degrades the material characteristics of the photosensitive body. To solve this problem, the first charging method is being replaced with the second method. The second charging method has the advantage of requiring a relatively low voltage power supply and greatly reducing the ozone.
The second charging method is disclosed in Published Unexamined Japanese Patent Application No. 56-91253. According to the second charging method, as shown in FIG. 6, first, second, and third charging rolers 2a, 2b, and 2c are arrange on a photosensitive drum 1 in contact with each other in the moving direction of the photosensitive drum (in the direction of arrow). As shown in FIG. 7, the charging rollers 2a, 2b, and 2c superimpose an alternating voltage on direct voltages of 200V, 350V, and 500V, respectively. The peak-to-peak value of the alternating voltage is 20% of each of the direct voltages. The voltages obtained by the superimposition are sequentially applied to the surface of the photosensitive drum 1. The potential of the surface of the photosensitive drum 1 gradually increases, and finally it is charged with a voltage of 500V necessary for exposure.
In the second charging method, however, since the direct and alternating voltages are applied to the charging rollers 2a, 2b, and 2c, two power supplies, i.e., direct and alternating power supplies are required, thus complicating an arrangement of the power supplies.